A fuel cell system is increasingly being used as a power source in a wide variety of applications. The fuel cell system has been proposed for use in vehicles as a replacement for internal combustion engines, for example. The fuel cell system may also be used as a stationary electric power plant in buildings and residences, portable power in video cameras, computers, and the like. Typically, the fuel cell system includes a plurality of fuel cells arranged in a fuel cell stack to generate electricity, which is used to charge batteries or provide power to an electric motor.
A typical fuel cell is known as a polymer electrolyte membrane (PEM) fuel cell, which combines a fuel such as hydrogen and an oxidant such as oxygen to produce electricity and water. The oxygen is generally supplied by an air stream. In order to perform within a desired efficiency range, a sufficient humidification of the polymer electrolyte membranes of the fuel cell should be maintained. The sufficient humidification desirably extends the useful life of the electrolyte membranes in the fuel cell, as well as maintains the desired efficiency of operation.
As part of the fuel cell system, a water vapor transfer (WVT) device may be employed to humidify the air stream entering the fuel cell stack. The WVT device transfers water vapor from an exhaust stream from the fuel cell stack to a feed stream entering the fuel cell stack. This is generally accomplished by using a water vapor transfer membrane which allows only water vapor to pass therethrough. This membrane is typically permanently attached to a diffusion media layer, called a separator, which controls gas flow.
An exemplary WVT device for a fuel cell system is disclosed in U.S. Pat. Appl. Pub. No. 2009/0092863 to Skala, the entire disclosure of which is hereby incorporated herein by reference. Skala describes a plate for a WVT device having a top layer formed from a diffusion medium and a bottom layer formed from a diffusion medium. An array of substantially planar elongate ribbons is disposed between the top and bottom diffusion medium layers to form the individual plate of the WVT device. A membrane is adhered to at least one of the top and bottom diffusion medium layers.
As part of a fuel cell system, the WVT device can be used to humidify an air stream entering the fuel cell stack. It is known to assemble the WVT device within a housing, and to incorporate the WVT assembly into a fuel cell module such as a lower end unit (LEU) of the fuel cell system. The WVT has both dry streams and wet streams passing through it. The dry stream is the air stream to a cathode inlet of the fuel cell system, and is generally pulled from the atmosphere via a compressor. The dry stream may have minimal RH with an oxygen content of approximately twenty-one percent (21%). The wet stream generally comes from a cathode outlet of the fuel cell stack, is highly humidified, and contains little to no oxygen. The exact RH and oxygen content is dependent on the operating conditions of the fuel cell stack. The pressure of the dry stream is typically higher than the wet stream, with the exact pressures depending on the pressure drop through module flow channels of the fuel cell system, and across the fuel cell stack for a given operating condition.
The dry stream and wet stream are kept separate using the membrane of the WVT device, which allows water vapor to pass from the wet stream to the dry stream without allowing gases such as oxygen through (i.e. the dry air flow is humidified but the oxygen content is not depleted). In order for the WVT device to function efficiently, both the wet stream and the dry stream must be sealed to their mating components within the fuel cell module. A dry stream leak results in loss of oxygen reactant delivered to the fuel cell stack, and requires additional air input by the compressor which reduces efficiency of the fuel cell system. A wet stream leak results in humidified air bypassing the WVT device, and either being dumped overboard or to exhaust. The wet stream leak results in less water vapor available for transfer to the dry stream.
Known designs have attempted to provide a substantially fluid tight seal for both the dry stream and the wet stream using elastomeric seals. In most cases, the sealing planes of the wet and dry streams have been perpendicular to each other. This has created issues such as the need to maintain tight tolerances between WVT assembly and the mating components of the fuel cell module in order to maintain targeted seal compression. Installation of the WVT assembly into the fuel cell module is also known to be difficult due to seals rubbing during installation. While it has also been possible to vary compression of the elastomeric seals to obtain sufficient sealing in two of the four sealing planes of the WVT assembly, it has been difficult to vary compression in the other two sealing planes of the WVT assembly.
There is a continuing need for a WVT assembly that simplifies installation into a fuel cell module, eliminates a use of compression sealing in desired sealing planes of the WVT assembly, and minimizes pressure differential across end plates of the WVT assembly. Desirably, the WVT assembly requires less structure and has a reduced part count in comparison to conventional assemblies, and allows for larger tolerances for the assembly interface with the fuel cell module.